Change out the Varible name to remove dashes.

stack/klipper.yml

@@ -4,7 +4,7 @@ services:
     hostname: klipper
     image: mkuf/klipper:latest
     devices:
-      - ${3d-printer-device}:${3d-printer-device}
+      - ${3dPrinterDevice}:${3dPrinterDevice}
     volumes:
       - /portainer/Files/AppData/Config/klipper/klipper/printer.cfg:/opt/printer_data/config/printer.cfg
       - /portainer/Files/AppData/Config/klipper/klipper/logs:/opt/printer_data/logs

template/apps/klipper.json

@@ -8,8 +8,8 @@
 	"env": [
 		{
 			"default": "/dev/ttymxc3",
-			"label": "3d-printer-device",
-			"name": "3d-printer-device"
+			"label": "3dPrinterDevice",
+			"name": "3dPrinterDevice"
 		}
 	],
 	"logo": "https://raw.githubusercontent.com/pi-hosted/pi-hosted/master/images/drupal.png",